1. Technical Field of the Invention
The present invention relates to plasma display screens, and more particularly to the control of the cells of such a display screen.
2. Description of Related Art
A plasma display screen is a screen of the matrix display type, formed from cells disposed at the interconnections of rows and columns. A cell comprises a cavity filled with a rare gas, and at least two control electrodes. In order to create a light point on the display screen using a given cell, the cell is selected by applying a potential difference between its control electrodes, then ionization of the gas in the cell is triggered generally by means of a third control electrode. This ionization is accompanied by an emission of ultraviolet rays. The creation of the light point is obtained by excitation of a red, green or blue light-emitting material by the emitted rays.
Conventionally, the control of a plasma display screen essentially comprises two phases, namely an address phase in which the cells (pixels) which will need to be lit and those that will need to be extinguished are determined, together with a display phase proper in which the cells having been selected in the address phase are effectively lit.
The address phase comprises a sequential selection of the rows of the matrix, generally a successive selection of the even rows followed by a successive selection of the odd rows. By way of example, a standby potential, for example 150 volts, is applied to the unselected rows, whereas an activation potential, for example 0 volts, is applied to a selected row. In order to select pixels chosen from the selected row, the pixels that will need to be lit in the display phase, a relatively high potential is for example applied to the corresponding columns of the matrix, for example 70 volts, via a power stage comprising MOS power transistors. Columns corresponding to the other pixels of the selected row, which will not need to be lit, have a potential of 0 volts applied to them. Thus, the cells of the activated row, which will need to be lit, see a column-row potential equal to around 70 volts whereas the other cells of this row see a column-row potential equal to 0 volts.
However, by applying different potentials to the rows of the matrix, it may also be envisaged in the address phase that a high potential be applied to a column in order to select a pixel which will need to be extinguished, and that a low potential be applied to a column in order to select a pixel which will need to be lit.
Reference is made to international patent application WO 02/15163, the disclosure of which is hereby incorporated by reference, which presents an example of the general operation of such a plasma display screen.
Conventionally, the selection and the deselection of the columns of the display screen are carried out by means of a selection and deselection signal according to a mode called “charge sharing effect” (CSE) mode.
During a selection when the column selection signal is emitted, according to the CSE mode, a part of the charges required originate from a capacitor incorporated within the display screen control circuitry.
In other words, when a column is selected, a part of the charges needed to generate the selection signal are transferred from the aforementioned capacitor towards the column to be selected. Once these charges have been transferred, the selection signal has reached a first intermediate plateau level value The output of the column driver circuit, coupled to the column in question, is then switched onto the power supply terminal of the circuit, in such a manner as to complete the amplitude of the selection signal up to its maximum value, generally the value of the power supply voltage.
Conversely, when the column is deselected, the output of the column driver circuit is firstly coupled to the charge capacitor so as to charge it up by returning a part of the charges to it. The deselection signal therefore goes from its maximum value to the intermediate plateau level value, then the column driver circuit output coupled to the column in question is switched towards the ground of the circuit, such that the deselection signal reaches its minimum value.
The CSE mode allows the power consumption of the circuit to be reduced since a part of the charge originates from a capacitor. The power gain with respect to a circuit operating without this mode can reach as much as 50%.
However, when the CSE mode is selected, certain pixels of the display screen having been addressed in order to be lit remain dark at the time of the ignition phase, in particular at the start of each scan of the display screen.
There is accordingly a need to provide a solution to this problem. There is a need to eliminate the appearance of dark pixels on the screen, in particular at the start of the display screen scan, while at the same time conserving the CSE mode allowing consumed power to be saved.